1. Field of the Invention
The present invention is in the field of cardiography, and more particularly relates to apparatus for producing at a high speed a printed record of electrocardiographic data played back from a magnetic tape at a much higher speed than the speed at which it was recorded.
2. The Prior Art
In ambulatory monitoring as currently practiced, the patient is outfitted with instruments for sensing electrocardial signals and with a small lightweight portable magnetic tape recorder for recording the sensed signals as the patient engages in various activities over an extended period of time, which may be as long as 26 hours. Following the recording session, the recorded magnetic tape is brought to a central facility where the recorded signals are studied to ascertain information about the behavior of the patient's heart.
Various types of equipment may be available at the central facility to facilitate study of the signals derived from the magnetic tape. At the minimum, a playback apparatus is required to reproduce the signals recorded on the tape. If necessary, the validator of the present invention can operate on signals produced by a relatively simple playback unit.
Another type of equipment typically used at the central facility is a playback-analyzer, which incorporates in a single unit of equipment both apparatus for playing back the tape and further includes circuitry for recognizing, counting, measuring, and displaying relevant characteristics of the recorded data signals. The validator of the present invention is preferably used in association with such a playback-analyzer. Such a playback-analyzer is described in U.S. Pat. No. Re. 29,921 for "Electrocardiographic Computer", reissued Feb. 27, 1979 to Cherry et al., and assigned to the assignee of the present invention. The disclosure of U.S. Pat. No. Re. 29,921 is incorporated herein by reference.
As described in U.S. Pat. No. Re. 29,921, the analysis portion of the equipment includes an arrhythmia computer which detects and digitally displays the number of premature ventricular contractions and superventricular ectopic beats, and generates a signal to cause a special mark to be written on the record when the arrhythmia occurrences exceed a predetermined number during a predetermined time interval. The same signal may also be used to slow the playback so that the arrhythmia occurrences can be plotted in real time. The arrhythmia computers include an ability to select one or more parameters to determine the occurrence of an arrhythmia; the parameters include paired beats, prematurity, width, or amplitude. Also, preselected abnormalities in heart function such as abnormal ventricular ectopic beats, superventricular ectopics, ST levels, rapid heart beat, slow heart rate, etc., may be recognized, so that the portion of the recording including the abnormalities can be plotted at real time speed.
The playback-analyzer equipment is not normally used to reproduce in printed form the entire recorded signal, although the equipment is capable of doing this. Instead, the playback-analyzer is normally used to recognize the occurrence of abnormal heart action so as to permit the ECG signals to be reproduced only for the time intervals when the abnormalities occurred. The plotted ECG signals are presented on a strip chart by the playback-analyzer.
The characteristics of the playback-analyzer have been reviewed in some detail so that the distinctions between the playback-analyzer and a validator can be seen more clearly and their complementary functions better understood.
As the word "validator" is currently used, it refers to a piece of equipment which enables an operator to verify the output of the analyzer portion of the playback-analyzer, e.g., to provide an independent check on the number of abnormalities of a particular type that have been recognized by the analyzer. In addition to, and in support of this basic function, validators typically are able to produce at very high speed a plot of the entire recorded ECG signal. Ideally, this complete record is formatted to facilitate visual detection of irregularities by the operator. The use of a well-designed format permits the operator to scan rapidly through the data, which typically might be printed on a dozen pages. The operator, based on his experience, recognizes the occurrences of abnormal heart action, and counts them, thus providing a completely independent check on the operation of the analyzer.
Several validators are known in the prior art. One such validator is disclosed in co-pending U.S. patent application Ser. No. 088,105 filed Oct. 24, 1979 for "Validator for Electrocardial Data Processing System", which is assigned to the assignee of the present invention. The validator described therein employs two memories, so that as one of the memories is receiving data from a playback apparatus, data is being supplied by the other memory to a mechanical plotter. The design of this validator assures that no part of the data is omitted from the plotted record, and the time of day is printed at intervals in the margin. The design is relatively sophisticated, and the unit is necessarily more expensive because of the extensive capability that it provides.
Another validator known in the art is the Model No. 7350 produced by AdvanceMed, 17346 Eastman Street, Irvine, Calif. This unit omits a portion of the ECG signal from the plot while the time of day characters are being printed This considerably reduces its value for use in verifying the accuracy of other equipment.
Thus, a need was recognized for a validator less expensive than that disclosed in the aforementioned U.S. patent application Ser. No. 088,105, but more accurate than the Model 7350 made by AdvanceMed. Cost considerations preclude the use of the dual memory technique, and suggest the use of a single-beam printer. However, it was by no means clear how a single beam could be employed to print the time-of-day characters without omitting a significant portion of the ECG signal. Multiplexing of the single beam was considered, but did not appear to be a feasible approach because of the high-beam scan rates required and the attendant reduction in brightness. A new approach was needed to satisfy the design requirements at minimal cost.